Nickel hydroxide is normally obtained by the reaction of nickel salts with alkali metal hydroxides. The nickel starting solutions are obtained in a first stage by digestion of metallic nickel, for example with HNO.sub.3. The hydroxide is obtained in a second stage by subsequent precipitation with alkali metal hydroxides. The disadvantages of these processes are, on the one hand, the poor filterability of the nickel hydroxide gel formed. The neutral salts formed and the excess alkali metal hydroxide are also very difficult to remove from the precipitate. Non-stoichiometric basic nickel salts are precipitated in particular from nickel chloride and/or sulfate solutions and are obstacles to conversion into pure hydroxide. In addition, stoichiometric quantities of neutral salts are unavoidably formed in the hydroxide precipitation step and have to be disposed of through the wastewater.
The electrolytic preparation of pure nickel hydroxide from metallic nickel in aqueous solutions of alkali metal salts is described in Italian patent 366 495. The unwanted formation of basic salts is mentioned in this document, too, and is counteracted by elaborate and hence uneconomical equipment-related measures.
The formation of nickel hydroxide in electrolytic processes has also been repeatedly observed as a secondary reaction and is described, for example, in J. Elektrochem. Soc., Vol. 137, No. 6, 1990, pages 1696 to 1702. According to this reference, the anodic formation of an Ni(OH).sub.2 layer on nickel electrodes was observed in voltametric measurements in sulfuric acid sulfate electrolyte solutions containing potassium chloride. The experimental results in this reference provide no indication as to how pure nickel hydroxide can be electrochemically produced on an industrial scale.
The object of the present invention is to provide an ecologically safe process for the production of nickel hydroxide which does not have any of the disadvantages of the described prior art.